Scanned electron beam columns are utilized in a variety of applications, including scanning electron microscopes, electron beam lithography, and electron beam inspection systems, etc. The electron beam source of the present invention was developed primarily for utilization in an electron beam lithography system in which one or more beams are selectively deflected to directly write circuit patterns on a semiconductor wafer located in a target plane. Such systems are described and illustrated, for example, in U.S. Pat. Nos. 4,390,789 (Smith et al) and 4,110,622 (Trotel); and in co-pending U.S. patent application Ser. No. 06,749,796, filed concurrently herewith and entitled "Multiple Channel Electron Beam Optical Column Lithography System And Method Of Operation" by Kenneth J. Harte. The disclosures in these patents and patent application are expressly incorporated herein in their entirety by this reference. Although the present invention was developed primarily with electron beam lithography systems in mind, it is to be understood that the invention has applicability in substantially all scanned electron beam systems wherein a relatively small beam spot is scanned or selectively deflected across a target plane.
Conventional electron beam guns utilized in scanned beam systems employ thermionic cathodes as the source of electrons. Thermionic cathodes are heated to the required temperature, generally by means of a connected filament through which a high current is passed to effect electron emission. In spite of their wide use, numerous disadvantages are associated with thermionic cathodes. Specifically, such cathodes are characterized by a low efficiency, with only one percent or less of the total emission current being utilized at the target. The emission current must therefore be quite high in order to provide a reasonable current at the beam spot location. This results in significant heating of stripping apertures along the beam column, thereby creating thermal expansion problems and requiring long warm-up times. In addition, the high emission current results in relatively high energy spread from electron-to-electron interactions, thereby creating lens and deflector chromatic aberrations. Moreover, the high emission current inherently creates a large number of backscatter or secondary electrons, thereby disposing a significant number of free electrons in the column during blanking of the beam. Consequently, there is significant contamination by cross-linked residual hydrocarbons, resulting in charging and unstable operation.
In addition to their low efficiency, thermionic cathodes are characterized by non-uniform current density across the beam, resulting in uneven effects produced across the beam spot area in the target plane. Further, beam blanking by the required electron optical means is relatively complex and is characterized by a relatively long transition time.
It is known in the prior art to employ a photoemitter cathode (or photocathode) as a beam source to expose a large area pattern on a target plane. Examples of systems of this type may be found in U.S. Pat. Nos. 4,039,810 (Heritage) and 4,227,090 (Amboss). These systems provide a large photocathode which emits a beam shaped to the desired circuit pattern in its entirety. The photocathode is irradiated by a light source through a mask bearing the features of the desired circuit pattern; alternatively, the emissions from the photocathode are optically projected by a reticle. The entire emission pattern is then electron-optically imaged onto the target. This approach has certain disadvantages, including the need to change the photocathode assembly, or the mask, in order to change the pattern to be exposed. In addition, from the electron-optical point of view, it is difficult to image relatively wide or large fields with uniform resolution across the entire field. Moreover, relatively large diameter lenses are required to effect the required focusing.